1. Field of the Invention
The present invention relates to a multiuser interference cancellation apparatus and, more particularly, to a multiuser interference cancellation method which ensures improved demodulation characteristics in a code division multiple access (CDMA) system even if the number of users simultaneously gaining access to the system is large.
2. Description of the Prior Art
In a CDMA system, the problem of a deterioration in demodulation characteristics due to interference signal components becomes more serious with the increase in the number of users simultaneously gaining access to the system. A multiuser cancellation method has been studied as a method for ensuring improved demodulation characteristics even if the number of users simultaneously gaining access is increased.
Japanese Patent Laid-open No. 11-205286 discloses an error transmission type of multiuser time/space interference canceller which is a receiver having both two functions: the function of canceling interference due to an antenna directivity, and the function of canceling interference based on regeneration of interference replicas, and having improved ability to eliminate interference. This multiuser time/space interference canceller is of such a type that the increase in the number of pieces of hardware is comparatively small, and is therefore practical.
This multiuser time/space interference canceller will be described with reference to FIGS. 1, 6, and 7. FIG. 1 shows the configuration of this multiuser time/space interference canceller. As shown in FIG. 1, the multiuser time/space interference canceller is constituted by antennas 1-1 to 1-N, interference estimation units (IEUs) 2-1 to 2-K, 3-1 to 3-K, and 4-1 to 4-K, and adders 5-1 to 5-N, and 6-1 to 6-N. N (N: an integer larger than or equal to 1) represents the number of elementary antennas, and K (K: an integer larger than or equal to 1) represents the number of users.
Signals received by the antennas 1-1 to 1-N are input to the interference estimation units 2-1 to 2-K in the first stage. The interference estimation units 2-1 to 2-K output interference replicas and symbol replicas in correspondence with the user signals. The interference replicas are input to the adders 5-1 to 5-N, while the symbol replicas are transmitted to the interference estimation units 3-1 to 3-K in the second stage. The adders 5-1 to 5-N respectively subtract the interference replicas from the antenna received signals in correspondence with the antennas 1-1 to 1-N and output the subtraction results to the interference estimation units 3-1 to 3-K.
The interference estimation units 3-1 to 3-K in the second stage generate interference replicas and symbol replicas and output the interference replicas to the adders 6-1 to 6-N and the symbol replicas to the interference estimation units 4-1 to 4-K in the third stage, as do the interference estimation units 2-1 to 2-K in the first stage. The interference estimation units 4-1 to 4-K in the third stage output demodulated signals.
FIG. 6 shows the configuration of each of the interference estimation units 2-1 to 2-K in the first stage, and FIG. 7 shows the configuration of each of the interference estimation units 3-1 to 3-K, and 4-1 to 4-K in the subsequent stages. In each of the interference estimation units 2-1 to 2-K in the first stage having the configuration shown in FIG. 6, the antenna received signals are despread by despread means 111-1 to 111-N on a RAKE path basis to be converted into baseband signals. The despread signals are directivity-controlled by a beam former 112.
FIG. 4 shows details of the configuration of the beam former 112. Antenna weights are prepared with respect to each stage and each RAKE path. Antenna weights wm,l,1 to wm,l,N are prepared with respect to the antenna received signals in the l-th path (l=1 to L) at the m-th stage (m=1 to M). The antenna weights are converted into the respective complex conjugates by complex conjugate generation means 51-1 to 51-N. The complex conjugates of the antenna weights and the antenna signals are multiplied together by multipliers 50-1 to 50-N, the multiplication results are input to an adder 52, and the result of combining by the adder 52 is obtained as an output from the beam former 112.
From the output from the beam former 112, a channel distortion is estimated by a channel estimation section 113. The estimated value is converted into the complex conjugate by a complex conjugate generation means 114. The output from the beam former 112 and the complex conjugate of the channel estimated value are multiplied together by a multiplier 115. An output from the multiplier 115 with respect to each RAKE path is input to a RAKE combining section 118. The RAKE combining section 118 generates a demodulated signal about which a hard decision is made by a hard decision means 119.
On the other hand, the outputs from the despread means 111-1 to 111-N are also input to an antenna weight adaptive updating section 117. The antenna weight adaptive updating section 117 performs incoming direction estimation based on the antenna received signals to obtain steering vectors with respect to incoming directions, and sets the steering vectors as antenna weights while normalizing them so that the beam gain (peak) is 1.
The antenna weight adaptive updating section 117 notifies the beam former 112 of the obtained antenna weights. An output from the hard decision means 119 is processed with respect to each path to generate interference replicas. A multiplier 123 multiplies the output from the hard decision means 119 by the output from the channel estimation section 113 and transmits a signal obtained as a symbol replica by this multiplication to the following stage. A multiplier 124 multiplies the multiplication result from the multiplier 123 by an interference suppression coefficient α and outputs the result of this multiplication to an antenna signal regeneration section 125.
FIG. 8 shows details of the configuration of the antenna signal regeneration section 125. Multipliers 160-1 to 160-N of the antenna signal regeneration section 125 regenerate antenna signals by multiplying the input signal by coefficients which are obtained by multiplying the antenna weights wm,l,1 to wm,l,N by an antenna gain correction coefficient β=N. The signals obtained as antenna signals by conversion in the antenna signal regeneration section 125 are respread by respread means 126-1 to 126-N in correspondence with the antennas 1-1 to 1-N and are added together with respect to the RAKE paths by adders 127-1 to 127-N to generate interference replicas.
The interference estimation units 3-1 to 3-K will be described with reference to FIG. 7. In the arrangement shown in FIG. 7, components identical or corresponding to those shown in FIG. 6 are indicated by the same reference characters. Symbol replicas transmitted from the interference estimation units 2-1 to 2-K in the first stage and residue signals output from the adders 5-1 to 5-N shown in FIG. 1 are input to the interference estimation units 3-1 to 3-K in the second stage. The residue signals are despread by despread means 111-1 to 111-N and directivity-controlled by a beam former 112. As antenna weight coefficients used in the beam former 112, the values obtained by the interference estimation units 2-1 to 2-K in the first stage are used without being changed.
An adder 128 adds together an output from the beam former 112 and the results of multiplication performed by a multiplier 129, i.e., multiplication of the symbol replica transmitted from the preceding stage by a coefficient [1−(1−α)m−1] determined by the interference suppression coefficient α and the stage number m. An output from the adder 128 is input to a channel estimation section 113. A channel estimated value is thereby obtained and is converted into the complex conjugate by a complex conjugate generation means 114. The output from the adder 128 and the complex conjugate of the channel estimated value are multiplied together by a multiplier 115. A RAKE combining section 118 combines outputs from the multipliers 115 obtained in correspondence with RAKE paths to obtain a demodulated signal. An output from a hard decision means 119 with respect to the output from the RAKE combininb section 118 and the channel estimated value output from the channel estimation section 113 are multiplied together by a multiplier 123 and the result of this multiplication is transmitted as a symbol replica to the following stage. This operation is the same as that in the arrangement shown in FIG. 6.
From the symbol replica output at the present time from the multiplier 123, the symbol replica transmitted from the preceding stage is subtracted by an adder 132, and the symbol replica difference output from the adder 132 is multiplied by the interference suppression coefficient α by a multiplier 124. An output from the multiplier 124 is converted into antenna signals by an antenna signal regeneration section 125. The regenerated antenna signals are respread by respread means 126-1 to 126-N and are added together by adders 127-1 to 127-N with respect to the RAKE paths to be transmitted as interference replicas to the adders 6-1 to 6-N shown in FIG. 1.
The interference estimation units 4-1 to 4-K do not perform interference replica regeneration. The output from the RAKE combininb section 118 shown in FIG. 7 is obtained as a demodulation result of each of the interference estimation units 4-1 to 4-k. The operation before the output from the RAKE combininb section 118 is the same as that in the interference estimation units 3-1 to 3-K in the second stage, and the description for it will not be repeated.
The above-described conventional apparatus for canceling multiuser interference has problems described below. The interference estimation units in the second and other subsequent stages are supplied with the residue signals and cannot use the antenna received signals. In the interference estimation units in the second and third stages, therefore, updating of antenna weights cannot be performed.
In the case where adaptive updating is used for generating antenna weights, the beam gain is not always equal to 1 and it is not possible to regenerate antenna signals accurate in level.